Method for improving the efficiency of absorption heat pumps using a crystallization-inhibiting additive

ABSTRACT

An absorption heat pump achieves improved efficiency by lowering the low cycle temperature of the circulation fluid. This is accomplished by adding a crystallization-inhibiting compound to the circulation fluid which substantially depresses the temperature at which the absorbent salt in the fluid begins to crystallize.

This application is a division of Ser. No. 09/193,520 filed Nov. 17,1998, now U.S. Pat. No. 6,177,025.

FIELD OF THE INVENTION

This invention is directed to absorption heat pumps which achieveimproved efficiency by widening the difference between the high and lowtemperatures of the working fluid. In particular, the invention isdirected to additives which depress the crystallization/precipitationtemperature of salt contained in the working fluid.

BACKGROUND OF THE INVENTION

In heat pumps of the absorption type, an absorbent, diluted with anabsorbed refrigerant, is heated in a generator to vaporize some of therefrigerant. The refrigerant vapor then flows to a condenser where it iscondensed to a liquid by heat exchange with an external cooling fluidmaintained at a low temperature by a heat sink. The liquefiedrefrigerant then flows through a valve to an evaporator which vaporizesthe refrigerant (usually at low pressure) to produce refrigeration.

The vaporized refrigerant then flows to an absorber where it is absorbedby concentrated absorbent supplied from the generators From theabsorber, the diluted absorbent passes to the generator where it isconcentrated by heating to vaporize some of the refrigerant, and thusrepeat the cycle.

Conventional absorption heat pumps typically employ an aqueous solutionof lithium bromide as an absorbent and water as a refrigerant. Theoperating efficiency of these heat pumps increases with the differencebetween the highest fluid temperature where the solution is dilute inlithium bromide and water is being vaporized, and the lowest fluidtemperature where the solution is very concentrated in lithium bromideand water is being absorbed. When operating in a refrigeration/airconditioning mode, the high temperature is fixed by the ambienttemperature. When operating in a temperature boosting mode, the hightemperatures can reach 98° C., 177° C., and 232° C. for single, doubleand triple effect machines, respectively. Some of these machines aredescribed in Herold and Radermacher “Absorption Heat Pumps”, MechanicalEngineering, August 1989, pp.68-73. Since the high cycle temperature isgenerally fixed by the application and/or pump type, the efficiency ofthe cycle can be increased by lowering the low cycle temperature.

As the low cycle temperature is reduced in an air conditioningapplication, the concentration of lithium bromide must be increased inorder to permit the continued absorption of water vapor. As the saltconcentration is increased and the temperature is decreased, asolubility limit is approached. If the solubility limit of lithiumbromide in water is exceeded, hydrated salt crystals may form whichblock the flow circulation in the absorber, rendering it useless. Thus,conventional absorption heat pumps use solutions containing about 60-62%salt, and operate at a minimum fluid temperature of about 4-7° C. in airconditioning applications. For heating applications, the saltconcentration may be lowered, to prevent freezing of the solution attemperatures down to −25° C. or lower.

Absorption heat pumps have many large-scale uses in industrialair-conditioning and refrigeration, as well as heating and temperatureboosting. There is always a need or desire for more efficient heat pumpswhich maximize the difference between the high and low fluidtemperatures at different parts of the cycle.

SUMMARY OF THE INVENTION

The present invention is an absorption heat pump which achieves agreater difference between the high and low fluid temperatures of thecirculation fluid. by reducing the minimum fluid temperature to levelsnot previously contemplated. Additives have been discovered whichinhibit the crystallization and precipitation of lithium bromide fromwater at concentrations of 60-62% lithium bromide and temperatures belowabout 4° C. without adversely affecting 1) the heat capacity of thesolution, 2) the solution rheological properties, 3) the solutiondiffusion or mass transfer coefficients, or 4) the ability of thesolution to absorb water vapor and transfer heat in the process. In anabsorption cycle, these additives permit operation at a lower lowtemperature, thereby improving the efficiency of the cycle. Theincreased efficiency makes the absorption heat pump more cost effectivecompared to conventional refrigeration technologies.

The additives for the aqueous lithium bromide solution can reduce theminimum low fluid temperature from about 4-7° C. to about 0° C. orlower. Some of the additives can reduce the minimum low fluidtemperature to −5° C. or lower, to −8° C. or lower, or even to −10° C.or lower. The additives can also be used to reduce the minimum low fluidtemperature in applications using lower concentrations of lithiumbromide in water.

Suitable additives are those which form complexes with lithium and/orbromine ions in aqueous solution. The additives and complexes formedmay 1) decrease the crystallization driving force, causingsupersaturation, 2) increase the critical supersaturation needed foreffective nucleation, and/or 3) decrease the crystal growth rate. Usefuladditives include compounds which form complexes with the lithium andbromine ions in solution, and which alter the surface energy of crystalembryos formed in solution just prior to nucleation.

With the foregoing in mind, it is a feature and advantage of theinvention to provide an aqueous lithium bromide solution useful inabsorption heat pumps, which has a wider range of cycle temperature dueto a lower minimum temperature for the onset of crystallization.

It is also a feature and advantage of the invention to provide anabsorption heat pump having greater efficiency due to a wider range ofcycle temperatures and a lower minimum cycle temperature.

The foregoing and other features and advantages will become furtherapparent from the following detailed description of the presentlypreferred embodiments, read in conjunction with the examples. Thedetailed description and examples are merely illustrative rather thanlimiting, with the scope of the invention being defined by the appendedclaims and equivalents thereof.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

In accordance with the invention, an absorption heat pump is providedwhich is operable using an aqueous circulation fluid including ahygroscopic salt as an absorbent material and water as a refrigerant.The preferred hygroscopic salt includes lithium bromide, alone or incombination with one or more additional absorbent materials or aids. Thesolution should contain about 30-85% by weight total absorbent material,preferably about 40-75% by weight, more preferably about 55-65% byweight. In practice, the maximum absorbent salt concentration will bethe concentration at saturation at the lowest temperature experienced bythe solution during operation.

The salt concentration must, at minimum, be sufficient to effectivelyabsorb the refrigerant at the lowest cycle temperature. The minimum saltconcentration useful for this purpose increases as the solutiontemperature is lowered, until a low temperature is reached where theminimum and maximum concentrations converge at a saturation level. For asolution employing only lithium bromide as the absorbent and only wateras the refrigerant, an optimum salt concentration of about 60-62% yieldsboth absorption and saturation at a minimum operating temperature ofabout 4-7° C.

The present invention is directed primarily toward lowering the minimumoperating temperature of solutions containing lithium bromide and water,to levels below the conventional minimum temperatures. To this end,crystallization-inhibiting compounds of selected types and amounts areadded to the aqueous solution of lithium bromide and water. Theadditives are present in low concentrations, typically up to about 5000molar ppm, more typically about 200-2000 molar ppm, most typically about500-1500 molar ppm. The term “molar ppm” is based on the amount oflithium bromide in the solution. For instance, an additive level of 500molar ppm means the solution contains 500 moles ofcrystallization-inhibiting additive for every million moles of lithiumbromide.

It is desirable to employ crystallization-inhibiting additives that areeffective at such low concentrations. This way, the additives have nosignificant effect on 1) the heat capacity of the solution, 2) thesolution rheological properties, 3) the solution diffusion or masstransfer coefficients, or 4) the ability of the solution to absorb watervapor and transfer heat. As explained above, thecrystallization-inhibiting additives are effective for lowering theminimum operating temperature of a solution containing 60-62% by weightlithium bromide (i.e. the onset of crystallization) to no more thanabout 0° C., preferably no more than −5° C., more preferably no morethan −8° C., most preferably no more than −10° C.

On the other hand, it has been found that increasing the concentrationof the crystallization inhibitor lowers the freezing point of thesolution. Thus, it is desirable to use as much of the additive aspossible without compromising the above properties or exceeding thesolubility limit for the additive.

The crystallization-inhibiting additives useful for lowering the onsetof crystallization have the tendency to form complexes with, andincrease the surface energy of embryos formed just prior to nucleation.They include without limitation salts of bromine; salts of alkali metalsincluding phosphates, chlorates, bromates, iodates, ferrocyanides,chlorides and the like; and organic compounds including crown ethers,dicarboxylic acids, tetracarboxylic acids, diphosphoric acids,diphosphonic acids, polyphosphoric acids, phosphates, formamides and thelike; and combinations including one or more of the foregoing. Specificcompounds found useful include potassium bromate, potassiumferrocyanide, ethylene diamine tetraacetic acid (EDTA), phosphoric acid,malonic acid, malic acid, potassium iodate. adenosine triphosphate(ATP), adenosine diphosphate (ADP),5-amino-2,4,6-trioxo-1,3-perhydrodizine-N,N-diacetic acid(uramil-N,N-diacetic acid), polyphosphoric acid (poly PA),1-hydroxyethlidene-1,1-diphosphonic acid (HEDP), diethylene triaminepenta (methylene phosphonic acid) (DTPMP), amino tri (methylenephosphonic acid) (ATMP), pyrophosphoric acid (PPA). methylenediphosphoric acid (MDPA), and combinations including one or more ofthem. Preferred additives include uramil N,N-diacetic acid, HEDP, DTPMP,ATMP, PPA, MDPA, and combinations thereof, at the concentrations statedabove. The crystallization-inhibiting additive can simply be added inwith the solution of refrigerant and absorbent.

The present invention is also directed to lowering the minimum operatingtemperatures of solutions containing lithium bromide in water, in thesame or lower amounts of lithium bromide, and with or without otheradditives. For some applications (e.g. heating), lithium bromide may bepresent at levels of about 30-60% by weight, desirably about 40-50% byweight. Other additives may include, for instance, corrosion inhibitors(for example, molybdate) and alkaline treating agents (for example,lithium hydroxide). Other additives known in the art as anti-foamingagents include alcohols and glycols as described, for example, in U.S.Pat. Nos. 3,276,217; 3,553,136; 3,609,087; 3,643,455; and 3,783,631.Other additives also include other bromine salts as described in U.S.Pat. No. 3,004,919; other lithium salts as described in U.S. Pat. Nos.3,316,728, 3,316,736, and 3,296,814; molybdates, borates and silicatesas described in U.S. Pat. No. 5,547,600; amines as described in U.S.Pat. No. 5,577,388; thiocyanates as described in U.S. Pat. No.5,653,117; and/or other additives known in the art.

For solutions containing lower amounts of lithium bromide, and/orcompounds other than lithium bromide and water, the minimum operatingtemperature of the starting solution, as well as the effects of thevarious crystallization-inhibiting additives used in accordance with theinvention, may differ in magnitude from the temperatures stated abovefor solutions containing only lithium bromide and water. However, thegeneral practice of the invention is similar for these other solutions.The type and amount of the crystallization inhibitor should be selectedto reduce the minimum operating temperature (onset of crystallization)for the solution by more than about 3° C., preferably more than 8° C.,more preferably more than 10° C., most preferably more than 12° C.,compared to the minimum operating temperature existing without any ofthe crystallization-inhibiting compound.

EXAMPLES

Solutions of refrigerant (water) and absorbent (lithium bromide) wereprepared with and without various crystallization-inhibiting additives.Each solution contained between 60-61% by weight lithium bromide inwater. For each solution, the onset of crystallization temperature wasmeasured by placing test tubes of the solution (initially at ambienttemperature) in a cooling bath programmned for a cooling rate of 20° C.per hour. The temperature of each sample was measured as a function oftime. As the temperature was lowered to the onset of crystallization,the heat of crystallization was released causing a brief rise intemperature. The following Table 1 show the onset of crystallizationtemperature measured for each solution. The solutions are listed in theorder of increasing effectiveness of the crystallization inhibitor.

TABLE 1 Effects of Different Crystallization Inhibitors Average OnsetAmount of of % Crystallization Inhibitor Crystallization Example Li BarInhibitor (molar ppm)* (° C.)  1 60.82 None — 4.38** (control)  2 60.54None — 3.30* (control)  3 60.82 TlCl₃ 500 3.17  4 60.54 KBr 589 2.75  560.54 C₂H₅NO 500 2.68  6 60.54 ZnBr₂ 500 2.50  7 60.82 C₂H₄O₂ (oxalic500 1.28 acid)  8 60.54 KBrO₃ 575 0.40  9 60.54 K₄Fe (CN)₆ 500 −0.01 1060.82 ATP 250 −1.05 11 60.54 EDTA 500 −1.05 12 60.82 ADP 250 −1.05 1360.54 H₃PO₄ 500 −1.96 14 60.54 C₃H₄O₄ (malonic 500 −2.09 acid) 15 60.54C₄H₆O₅ (malic 500 −2.28 acid) 16 60.54 KIO₃ 250 −4.31 17 60.82 Poly PA500 −4.73 18 60.54 Uramil N,N- 500 −5.69 diacetic acid 19 60.82 HEDP 500−6.15 20 60.82 DTPMP 500 −6.67 21 60.82 ATMP 500 −8.24 22 60.82 PPA 500−8.52 23 60.54 MDPA 250 −10.16 *Molar ppm refers to moles of inhibitorper one million moles of LiBr. **The solution of Example 1 had anequilibrium freezing temperature of 11.86° C. The solution of Example 2had an equilibrium freezing temperature of 12.99° C.

Some of the additives were tested at different concentrations to showthe effect of different additive levels on the initial freezing point(onset of crystallization) of the aqueous lithium bromide solutions. Asshown in the following Table 2, the onset of crystallization generallybecomes lower as the additive levels are increased. However, thecorrelation is not linear, and the onset of crystallization appears toapproach a different low limit for each additive.

TABLE 2 Effects of Different Concentrations of Various CrystallizationInhibitors Additive Additive Concen- Crystallization Concen-Crystallization tration Temperature tration Temperature (mole Tmin ±Std. (mole Tmin ± Std. PPM #) Deviation (° C.) PPM #) Deviation (° C.)MDPA (Methylene Diphosphoric PPA (pyrophosphoric acid acid CH₆P₂O₆) in60.82% LiBr H₄P₂O₇) in 60.82% LiBr 0    4.65 ± 0.56° C. 0    4.65 ±0.56° C. 50  −4.80 ± 3.67° C. 200  −9.55 ± 3.83° C. 150  −2.63 ± 5.11°C. 500  −8.52 ± 4.17° C. 250  −8.88 ± 3.17° C. 750  −8.89 ± 7.96° C.ATMP DTPMP (Diethylene triamine (aminotri(methylenephosphonicpenta(methylene phosphonic acid) acid)N(CH₂PO₃H₂)₃) in 60.82%CH₂PO₃H₂N(C₂H₄N(CH₂PO₃H₂)₂)₂) LiBr in 60.82% LiBr 0    4.65 ± 0.56° C. 0   4.65 ± 0.56° C. 200  −5.93 ± 3.62° C. 200 −12.72 ± 5.56° C. 500 −8.24 ± 2.25° C. 500  −6.67 ± 6.79° C. 1500 −14.22 ± 0.74° C. 1500−13.47 ± 2.20° C. HEDP (1-Hydroxyethlidene-1,1- Uramil-N,N-diaceticacid, also called diphosphonic acid 5-Amino-2,4,6-trioxo-1,3-CH₃C(OH)(PO₃H₂)₂) in 60.82% perhydrodizine-N,N-C₈H₉O₇N₃) in LiBrdiacetic acid 60.82% LiBr 0    4.65 ± 0.56° C. 0    4.65 ± 0.56° C. 200−10.97 ± 6.61° C. 200  −2.36 ± 2.69° C. 500  −6.15 ± 7.49° C. 500  −4.69± 3.73° C. 1500 −13.01 ± 1.71° C. 1000  −6.10 ± 3.30° C. 1500  −7.50 ±2.38° C.

While the embodiments of the invention described herein are presentlyconsidered, various modifications and improvements can be made withoutdeparting from the spirit and scope of the invention. The scope of theinvention is indicated by the appended claims, and all chances that fallwithin the meaning and range of equivalents are intended to be embracedtherein.

We claim:
 1. A method for improving the efficiency of an absorption heatpump comprising the step of: circulating an aqueous circulation fluid inthe absorption heat pump, the aqueous circulation fluid including anabsorbent salt and a crystallization-inhibiting additive in the range ofabout 200 to 5000 molar ppm, selected from the group consisting of5-amino-2,4,6-trioxo-1,3-perhydrodizine-N,N-diacetic acid (uramilN,N-diacetic acid), 1-hydroxyethlidene-1,1-diphosphonic acid (HEDP),diethylene triamine penta (methylene phosphonic acid) (DTPMP), amino tri(methylene phosphonic acid) (ATMP), and combinations including one ormore of the foregoing, the circulation fluid having a high cycletemperature and a low cycle temperature, wherein the low cycletemperature is not higher than −5° C. at an absorbent salt concentrationof about 30-85% by weight.
 2. A method for improving the efficiency ofan absorption heat pump comprising the step of: circulating an aqueouscirculation fluid in the absorption heat pump, the aqueous circulationfluid including about 30-85% by weight lithium bromide salt and in arange of about 200 to about 2000 molar ppm of acrystallization-inhibiting additive selected from5-amino2,4,6-trioxo-1,3-perhydrodizine-N,N-diacetic acid (uramilN,N-diacetic acid), 1-hydroxyethlidene-1,1-diphosphonic acid (HEDP),diethylene triamine penta (methylene phosphonic acid) (DTPMP), amino tri(methylene phosphonic acid) (ATMP), and combinations including one ormore of the foregoing.
 3. The method of claim 2, wherein the additive ispresent at a level of about 500-1500 molar ppm.
 4. The method of claim2, wherein the circulation fluid comprises about 40-75% by weight of thelithium bromide salt.
 5. The method of claim 2, wherein the circulationfluid comprises about 55-65% by weight of the lithium bromide salt. 6.The method of claim 1, wherein the circulation fluid has a low cycletemperature not higher than −8° C. at said absorbent salt concentration.7. The method of claim 1, wherein the circulation fluid has a low cycletemperature not higher than −10° C. at said absorbent saltconcentration.
 8. The method of claim 1, wherein the circulation fluidcomprises about 40-75% by weight of the absorbent salt.
 9. The method ofclaim 1, wherein the circulation fluid comprises about 55-65% by weightof the absorbent salt.
 10. The method of claim 1, wherein the absorbentsalt at comprises lithium bromide.
 11. The method of claim 1, whereinthe crystallization-inhibiting additive is present at a level in a rangeof about 200 to about 2000 molar ppm.
 12. The method of claim 1, whereinthe additive is present at a level of about 500-1500 molar ppm.
 13. Amethod for improving the efficiency of an absorption heat pumpcomprising the step of: circulating an aqueous circulation fluid in theabsorption heat pump, the aqueous circulation fluid including anabsorbent salt and a crystallization-inhibiting additive in the range ofabout 200 to 5000 molar ppm, said crystallization-inhibiting additivecomprising 5-amino-2,4,6-trioxo-1,3-perhydrodizine-N,N-diacetic acid(uramil N,N-diacetic acid), the circulation fluid having a high cycletemperature and a low cycle temperature, wherein the low cycletemperature is not higher than −5° C. at an absorbent salt concentrationof about 30-85% by weight.